The invention relates to a method and system for utilizing waste that contains fibres and combustible material. In addition, the invention relates to a method for processing fibre in a paper or board mill that utilizes recycled fibre pulp.
Conventionally, the options available in waste disposal and processing have been: transport to landfill sites, incineration in mass incineration plants and—to a limited extent—recovery of recyclable materials. The last-mentioned option generally requires separation of wastes at the source, wherein recyclable waste fractions, such as biowaste, paper and board, and glass and metals are separated and collected separately. However, source separation is always insufficient, which means that the amount of mixed waste is now, and will also be in future, considerable.
The form of energy recovery that has become popular recently is one in which recovered and/or refuse-derived fuel (REF, RDF) processed from waste is burned either in conventional boilers together with other fuel or as the main fuel in combustion plants specially designed for recovered/refuse-derived fuel. The wastes most suitable for energy recovery include packaging, paper and plastic wastes from industry and commerce as well as construction wastes, which can constitute as much as 70 to 80% of the amount of waste usually transported to landfills. Dry household waste can also be used in the production of energy provided that, among other things, metals, glass and biowaste have been separated from it first.
Most economically, recovered fuel is produced from a source-separated combustible waste fraction, i.e. the so-called energy fraction of waste. The processing stages of this REF fuel (recovered fuel) typically include removal of oversized pieces, crushing of waste, separation of metals as well as removal of sand and stones. Finished REF fuel contains predominantly plastics, wood, paper and board. The proportion of impurities in the fuel may be, for example, of the order of 5% depending on the sorting process. Recovered fuel can also be produced from unsorted mixed waste by mechanical handling processes, in which case the end product is called RDF fuel (refuse-derived fuel). A difference with respect to the handling of source-separated waste is constituted by a more mixed composition of the waste raw material, which is mainly shown as a higher biowaste content. Consequently, more sorting stages are needed in the production of RDF fuel than at an REF plant, for example, gravimetric separation stages, i.e. sorting stages based on the size and density of particles, whereby heavy matter, such as food scraps, can be efficiently separated from the fraction intended for combustion. The composition of finished RDF fuel is very much like that of REF fuel, but the proportion of impurities in it can be slightly higher, for example, of the order of 8%.
Energy recovery from waste should not be an end in itself, but rather aim for sensible macroeconomical reclamation of wastes or for making them harmless. In all reclamation of waste, the primary object should be recovery of materials contained in waste when it is economically profitable and, only secondarily, utilization of waste as fuel. Landfilling should be the very last option.
WO application publication 98/18607 discloses a process for treatment of waste that contains recyclable components, in which process waste is agitated in water employing mechanical force, whereby the size of pieces in waste is reduced and fibres are suspended in water. The heavy fraction containing metals, the lightweight fraction containing plastics and the fraction containing fibres are separated from the suspension in stages. The process is intended for treatment of packaging waste which contains different plastics and mixed materials, in particular board lined with plastic and/or metal foils, and metal cans. The fibre content of this kind of raw material is generally relatively low, wherefore the special problems associated with the recovery of fibres and with the quality of pulp have not been taken into consideration to a sufficient degree in the process. The publication has also failed to take into account matters that form an integral part of the recovery of fibres, such as, meeting of energy demand and circulation of waters.
The recovery of fibres from waste is also discussed, among other things, in GB patent 1,364,474 and in published GB application 2,026,019.